1. Technical Field of the Invention
The invention relates to a screw, in particular a concrete screw, according to the preamble of claim 1.
2. Prior Art
WO 98/04842 teaches a thread forming concrete screw which has a cylindrical core and a thread designed in a single piece with the core. Cutting notches are provided in the vicinity of the outer circumference, which are formed by triangles that are open towards the outer circumference of the thread. Radially of the central longitudinal axis, the flanks of the thread are substantially parallel to each other. The cutting notches are provided only along a section that extends from the pilot end of the screw counter to the lengthwise screw-in direction. This is followed by a thread section without cutting notches which has, however, parallel flanks and the same thread diameter. The concrete screw requires high screw-in moments, in particular for the concrete screw to be completely screwed in.
U.S. Pat. No. 5,827,030 teaches a thread-forming screw of the generic type which, starting from its tip, comprises a cutting thread with cutting notches and an adjoining standard thread of substantially triangular cross-sectional shape.
The gist of the invention resides in that a cutting thread is provided that forms an internal thread in a drilled hole. The cutting thread is adjoined by a standard thread that tapers off radially towards the outer circumference of the thread.
It is an object of the invention to further develop a concrete screw of the generic type so that it can be screwed in more easily without any loss of stability under tensile load.
This object is attained by the characterizing features of claim 1. The gist of the invention resides in that a cutting thread is provided that forms an internal thread in a drilled hole. The cutting thread is adjoined by a standard thread that tapers off radially towards the outer circumference of the thread.
The advantage of the further features resides in that the diameter DN of the standard thread is smaller than the diameter DS of the cutting thread so that, when the screw is screwed in, the friction exercised by the standard thread on the wall is inferior to the friction exercised by the cutting thread on the wall.